A method and apparatus for use in creating a magnetic declination profile for a borehole, which magnetic declination profile can be used to correct directional measurements made in the borehole with magnetic instruments.
Measuring devices and methods used in boreholes typically make use of one or more earth fields in order to provide measurements for the inclination and direction of a borehole and for the orientation of objects located in the borehole.
The inclination of a borehole is sometimes referred to as the xe2x80x9cdriftxe2x80x9d of the borehole and is an expression of the deviation of the borehole from vertical (i.e., from the direction of the earth""s gravity vector).
The direction of a borehole is sometimes referred to as the xe2x80x9cazimuthxe2x80x9d of the borehole and is an expression of the direction of the borehole in a horizontal plane relative to a calibration direction such as magnetic North or true North.
The orientation of a point or object in a borehole is sometimes referred to as the xe2x80x9ctoolfacexe2x80x9d of the point or object and is an expression of the orientation of the point or object in a plane perpendicular to the longitudinal axis of the borehole relative to a reference orientation.
A variety of measuring instruments have evolved for the measurement of inclination and direction of a borehole and the orientation of points or objects in the borehole.
Inclination measurements in a borehole are commonly made with instruments that are sensitive to the earth""s gravity field. Such gravity instruments include, for example, plumb bobs and accelerometers and are typically capable of measuring the amount of vertical deviation of the borehole but not the direction of the vertical deviation.
Directional measurements in a borehole are commonly made with instruments which are sensitive to the earth""s magnetic field. Such magnetic instruments include, for example, compasses and magnetometers. Magnetic instruments are typically capable of providing a measurement of the direction of the borehole in a borehole coordinate system, but are unable to convert this measurement to a direction or azimuth of the borehole in a more useful reference coordinate system which is defined by the direction of the gravity vector and by compass directions in a plane perpendicular to the gravity vector.
The reference coordinate system conventionally has its X-axis parallel to the earth""s surface and pointing North, its Y-axis parallel to the earth""s surface and pointing East, and its Z-axis perpendicular to the earth""s surface and pointing vertically down.
The borehole coordinate system is conventionally described as having its z-axis along the borehole axis, its y-axis parallel to the earth""s surface and its x-axis perpendicular to both the y and z axes.
As a result, measurements taken with instruments which are sensitive to the earth""s magnetic field are typically converted to values corresponding with the reference coordinate system in order to provide enhanced survey accuracy. Conventionally, this conversion is carried out by using data relating to the inclination of the borehole to transform data relating to the direction of the borehole in the borehole coordinate system to values expressed in the reference coordinate system.
Measurements of the orientation of a point or object in a borehole may be made with gravity instruments. For example, the xe2x80x9chigh sidexe2x80x9d or the xe2x80x9clow sidexe2x80x9d of a borehole can be determined with instruments which are sensitive to the earth""s gravity field.
Measurements of the orientation of a point or object in a borehole may also be made with magnetic instruments. Such measurements are sometimes referred to as xe2x80x9cmagnetic toolfacexe2x80x9d measurements and are essentially an expression of the orientation of the point or object relative to the x and y axes in the borehole coordinate system.
As a result of the above, borehole survey apparatus often include gravity instruments which are sensitive to the earth""s gravity field as well as magnetic instruments which are sensitive to the earth""s magnetic field so that the apparatus are capable of providing measurements of the inclination and direction of the borehole as well as the orientation of points or objects in the borehole.
Survey apparatus of the type described above are typically quite rugged and capable of enduring severe environmental conditions of vibration, heat and pressure. They are thus well suited to the rigors of use in downhole equipment such as drilling assemblies and measurement while drilling (MWD) systems.
Unfortunately, however, there are difficulties associated with the use of survey apparatus which include magnetic instruments. First, magnetic instruments take directional measurements relative to magnetic North instead of true North, with the result that magnetic directional measurements must be corrected so that they are expressed relative to true North. Such correction is typically performed empirically with the use of magnetic declination charts, thus complicating and contributing a source of error to the resulting corrected measurement.
Second, survey apparatus including magnetic instruments are also not well suited for use in circumstances where interference with the earth""s magnetic field is present. For example, magnetic instruments are not commonly used for borehole surveys in boreholes which are lined with metal casing.
Furthermore, when magnetic instruments are used in drilling assemblies or MWD systems, they are typically isolated from the interfering magnetic effects of the drilling string by being contained in non-magnetic housings and by being located adjacent to non-magnetic drill collars or drill pipe. Despite such measures, error remains present in magnetic instrument measurements due to the influence of magnetic deposits (or the casing of nearby boreholes) in the formation being drilled and due to the effects of magnetism in the drilling string and the drilling assembly.
The effects of magnetism in the drilling string and the drilling assembly are reasonably well understood and numerous methods have been developed for addressing the measurement error resulting from these effects. Examples of such methods are found in U.S. Pat. No. 4,682,421 (van Dongen et al), U.S. Pat. No. 5,103,177 (Russell et al), U.S. Pat. No. 5,435,069 (Nicholson), U.S. Pat. No. 5,787,997 (Hartmann) and U.S. Pat. No. 5,806,194 (Rodney et al).
Some efforts have also been made to address the issues associated with orienting objects in a borehole using magnetic instruments where magnetic deposits or adjacent casing strings are present. Typically these methods are used to avoid adjacent boreholes during perforation operations or during drilling of a borehole and may in fact utilize the interfering effects caused. by such adjacent boreholes. Examples of such methods are found in U.S. Pat. No. 3,704,749 (Estes et al), U.S. Pat. No. 3,964,553 (Basham et al), U.S. Pat. No. 4,593,770 (Hoehn) and U.S. Pat. No. 5,582,248 (Estes).
Many of the difficulties associated with the use of magnetic instruments to take measurements in boreholes may be overcome by using instruments which take measurements which are not influenced by magnetic flux in the borehole. These instruments typically measure changes of direction relative to a calibration direction. One example of such an instrument is a conventional gyroscope, which can be oriented in a calibration direction and will then sense movement relative to the calibration direction.
Such xe2x80x9cnon-magneticxe2x80x9d instruments may also utilize an earth field vector to assist in establishing the calibration direction, such as the earth""s inertial angular velocity vector as described in U.S. Pat. No. 4,433,491 (Ott et al). One example of a xe2x80x9cnon-magneticxe2x80x9d instrument which makes use of the earth""s inertial angular velocity vector is a xe2x80x9cnorth seekingxe2x80x9d gyroscopic instrument, which is capable of taking directional measurements relative to true North.
Gyroscopic instruments of a variety of types are used frequently to survey existing boreholes and are particularly suited for use in boreholes containing casing, since gyroscopic instruments are not influenced by magnetic flux and in particular by magnetic interference caused by metallic casing. Gyroscopic instruments are also capable of taking relatively accurate and reliable measurements.
Unfortunately, however, gyroscopic instruments are not particularly rugged and are thus not well suited for use in drilling assemblies or in MWD systems. As a result, the use of gyroscopic instruments for taking measurements in boreholes is generally confined to lowering the gyroscopic instrument in the borehole on a wireline to take measurements before a downhole operation in the borehole has taken place or after such an operation has been completed. The actual downhole operation is performed either without measurements being taken or with the use of survey apparatus including magnetic instruments which inherently provide errors due to discrepancies between true North and magnetic North and magnetic effects of nearby magnetic deposits or adjacent casing strings.
There is therefore a need for a method and apparatus for creating a magnetic declination profile for a borehole which will assimilate errors which are inherent in measurements made by magnetic instruments, so that directional measurements made with magnetic instruments can easily be corrected to provide reliable directional information pertaining either to the direction of the borehole or the orientation of a point or object in the borehole.
The present invention is a method and apparatus for creating a magnetic declination profile for a borehole. The magnetic declination profile may include a value of magnetic declination at as few as one longitudinal location in the borehole. Preferably, however, the magnetic declination profile includes values of magnetic declination at more than one longitudinal location in the borehole.
The method involves making a magnetic directional measurement at a longitudinal location in the borehole, wherein the magnetic directional measurement is influenced by magnetic flux in the borehole. The magnetic directional measurement is made at a magnetic measurement orientation. The method further involves making a reference directional measurement, wherein the reference directional measurement is not influenced by magnetic flux in the borehole. The reference directional measurement is made at a reference measurement orientation, wherein there is an orientation relationship between the magnetic measurement orientation and the reference measurement orientation and wherein the orientation relationship is known or is ascertainable. A value of magnetic declination at the longitudinal location can then be obtained using the magnetic directional measurement, the reference directional measurement and the orientation relationship.
The magnetic directional measurement may be any directional measurement which is influenced by magnetic flux and which is directed at determining the direction of magnetic vectors. Preferably the magnetic directional measurement is directed at determining directions relative to the earth""s magnetic field vector but includes the effects of interfering magnetic vectors which could influence the magnetic directional measurement.
The reference directional measurement may be any directional measurement which is not influenced by magnetic flux. Preferably the reference directional measurement is directed at determining changes of direction relative to a calibration direction, which calibration direction may be provided by an earth field vector such as the earth""s inertial angular velocity vector or may be provided in some other manner.
The value of magnetic declination that is obtained using the invention may be a representation of the difference between true North and magnetic North at the longitudinal location if no sources of magnetic interference are present at the longitudinal location. Usually, however, the value of magnetic declination is a representation of any and all magnetic influences at the longitudinal location of the type which will result in the magnetic directional measurement being different from the reference directional measurement when the magnetic measurement orientation is the same as the reference measurement orientation.
In one method aspect of the invention, the invention is comprised of a method for creating a magnetic declination profile for a borehole comprising the steps of:
(a) making a first magnetic directional measurement at a first longitudinal location in the borehole, wherein the first magnetic directional measurement is influenced by magnetic flux and wherein the first magnetic directional measurement is made at a first magnetic measurement orientation;
(b) making a first reference directional measurement, wherein the first reference directional measurement is not influenced by magnetic flux, wherein the first reference directional measurement is made at a first reference measurement orientation, wherein there is a first orientation relationship between the first magnetic measurement orientation and the first reference measurement orientation and wherein the first orientation relationship is known or ascertainable; and
(c) obtaining a value of magnetic declination at the first longitudinal location in the borehole using the first magnetic directional measurement, the first reference directional measurement and the first orientation relationship.
The first magnetic directional measurement may be made using any method or apparatus which is influenced by magnetic flux. Preferably the first magnetic directional measurement is made using a magnetic instrument. The magnetic instrument may be comprised of a compass. Preferably the magnetic instrument is comprised of a magnetometer. In the preferred embodiment the magnetometer is a triaxial magnetometer which is capable of measuring magnetic flux along three mutually perpendicular axes. The first magnetic directional measurement provides an indication of the direction of the resultant magnetic field vector at the first longitudinal location and may optionally also provide an indication of the magnitude of the resultant magnetic field at the first longitudinal location.
The first reference directional measurement may be made using any method or apparatus which is not influenced by magnetic flux. The first reference directional measurement may be made using a reference instrument which measures changes of direction relative to a calibration direction. The calibration direction may be related or unrelated to the earth""s inertial angular velocity vector. Preferably the reference directional measurement is made with a reference instrument which is comprised of a gyroscopic instrument.
The step of obtaining the value of magnetic declination may be comprised of the steps of calculating a first measurement differential between the first magnetic directional measurement and the first reference directional measurement, calculating a first orientation differential between the first magnetic measurement orientation and the first reference measurement orientation, and adjusting the first measurement differential by the amount of the first orientation differential to obtain the value of magnetic declination at the first longitudinal location in the borehole.
The first orientation differential may be any amount or value but is preferably equal to zero so that the first measurement differential is equal to the value of magnetic declination at the first longitudinal location in the borehole. The first orientation differential may be established by linking the magnetic instrument and the reference instrument.
The invention may be used to create a magnetic declination profile in any borehole. The invention is, however, particularly well suited for use in boreholes having an inclination relative to vertical of less than about five degrees, since values of magnetic declination in such boreholes cannot conventionally be obtained effectively by combining magnetic directional measurements with data from previous borehole surveys indicating borehole inclination and azimuth. As a result, preferably the method is used to obtain a value of magnetic declination where the inclination of the borehole at the first longitudinal location is less than about five degrees.
In addition, although the invention may be used to create magnetic declination profiles in open or uncased boreholes, the invention is particularly suited for use in boreholes containing a metallic casing. The presence of metallic casing in the borehole will result in magnetic interference which can be assimilated into the magnetic declination profile. As a result, preferably the method is used to obtain a value of magnetic declination where the borehole is lined with a metallic casing at or in the proximity of the first longitudinal location.
The method may be used to create a magnetic declination profile for the borehole at only the first longitudinal location. Preferably, however, the method is performed at a plurality of longitudinal locations in the borehole in order to create a magnetic declination profile for the borehole at the plurality of longitudinal locations.
For example, the method may be performed at a second longitudinal location in the borehole by performing the following steps:
(d) making a second magnetic directional measurement at a second longitudinal location in the borehole, wherein the second magnetic directional measurement is influenced by magnetic flux and wherein the second magnetic directional measurement is made at a second magnetic measurement orientation;
(e) making a second reference directional measurement, wherein the second reference directional measurement is not influenced by magnetic flux, wherein the second reference directional measurement is made at a second reference measurement orientation, wherein there is a second orientation relationship between the second magnetic measurement orientation and the second reference measurement orientation and wherein the second orientation relationship is known; and
(f) obtaining a value of magnetic declination at the second longitudinal location in the borehole using the second magnetic directional measurement, the second reference directional measurement and the second orientation relationship.
Similarly, the step of obtaining the value of magnetic declination at the second longitudinal location may be comprised of the following steps:
(d) calculating a second measurement differential between the second magnetic directional measurement and the second reference directional measurement;
(e) calculating a second orientation differential between the second magnetic measurement orientation and the second reference measurement orientation; and
(f) adjusting the second measurement differential by the amount of the second orientation differential to obtain the value of magnetic declination at the second longitudinal location in the borehole.
The second orientation relationship may be different from the first orientation relationship, but preferably the second orientation relationship and the first orientation relationship are the same so that the second orientation differential is equal to the first orientation differential.
In an apparatus aspect of the invention, the invention is comprised of an apparatus for use in creating a magnetic declination profile for a borehole comprising:
(a) a magnetic instrument for making a magnetic directional measurement which is influenced by magnetic flux;
(b) a magnetic orientation calibration indicator associated with the magnetic instrument for providing a magnetic instrument calibration orientation;
(c) a reference instrument for making a reference directional measurement which is not influenced by magnetic flux; and
(d) a reference orientation calibration indicator associated with the reference instrument for providing a reference instrument calibration orientation;
wherein the magnetic instrument and the reference instrument are linked such that a constant indicator differential can be maintained between the magnetic orientation calibration indicator and the reference orientation calibration indicator.
The reference instrument may be comprised of any apparatus or device which is capable of making a directional measurement which is not influenced by magnetic flux. Preferably the reference instrument is comprised of a gyroscopic instrument.
The magnetic instrument calibration orientation provided by the magnetic orientation calibration indicator may be referenced to any calibration direction, but is preferably referenced to some known direction relative to magnetic North. Most preferably, the magnetic orientation calibration indicator is configured so that the magnetic instrument indicates magnetic North when the magnetic orientation calibration indicator is pointed at magnetic North (in the absence of sources of magnetic interference).
The reference instrument calibration orientation provided by the reference orientation calibration indicator may be referenced to any calibration direction, but is preferably referenced to some known direction relative to true North. Most preferably, the reference orientation calibration indicator is configured so that the reference instrument indicates true North when the reference orientation calibration indicator is pointed at true North.
The magnetic orientation calibration indicator may be aligned with the reference orientation calibration indicator to provide any amount of indicator differential. Preferably, however, the magnetic orientation calibration indicator is aligned with the reference orientation calibration indicator such that the indicator differential is equal to zero.
The magnetic instrument and the reference instrument are each preferably contained in a housing, which housing adjacent to the magnetic instrument is preferably comprised substantially of a non-magnetic material so that the housing does not provide a source of magnetic interference. The housing may be comprised of a single housing section or may be comprised of a plurality of housing sections.
The apparatus may be lowered into the borehole in any manner, and may be incorporated into a pipe string. Preferably, however, the apparatus is lowered into the borehole on a wireline. The apparatus is preferably further comprised of a connector for connecting the apparatus to the wireline or to a pipe string.
Preferably the distance between the reference instrument and the magnetic instrument is minimized in order to reduce the likelihood of error due to misalignment of the instruments or due to bending or other deformation of the apparatus during use.
In the preferred embodiment, however, the reference instrument is separated longitudinally from the magnetic instrument to minimize the likelihood of either instrument interfering with the measurements of the other instrument. In the preferred embodiment, the amount of longitudinal separation between the instruments is preferably a convenient distance such as one meter or a multiple of one meter so that the position of the reference instrument in the borehole can easily be calculated from the position of the longitudinal locations at which magnetic measurements are made, and vice versa.
This in turn assists in the creation of the magnetic declination profile, particularly where the magnetic instrument and the reference instrument collect other data which is not directly related to the creation of the magnetic declination profile, since such other data may be used to provide a survey of the borehole which in turn may possibly be used to verify the measurements of the magnetic instrument and the reference instrument.
In a further method aspect of the invention, the invention is comprised of a method for conducting a magnetic declination survey for a borehole, the method comprising the following steps:
(a) connecting a magnetic instrument with a gyroscopic instrument to provide a magnetic declination logging tool;
(b) aligning a magnetic orientation calibration indicator associated with the magnetic instrument with a reference orientation calibration indicator associated with the gyroscopic instrument to provide a known first indicator differential between the magnetic orientation calibration indicator and the reference orientation calibration indicator;
(c) lowering the magnetic declination logging tool into the borehole to position the magnetic instrument at a first longitudinal location;
(d) making a first magnetic directional measurement with the magnetic instrument at the first longitudinal location;
(e) making a first reference directional measurement with the gyroscopic instrument; and
(f) obtaining a value of magnetic declination at the first longitudinal location in the borehole using the first magnetic directional measurement, the first reference directional measurement and the first indicator differential.
This further method aspect of the invention combines the features of the method and apparatus aspects of the invention described above.